This invention relates to optical detector cells and, more particularly, to optical flow cells capable of absorbance, turbidimetric, nephelometric and fluorescence measurements on a liquid.
Many analytical systems use optical detectors for measuring various physical properties of an analyte to be measured. These optical cells may be either fixed cells such as a cuvette or dynamic flow cells (in which a liquid is designed to flow through the cell). In both cases the liquid is subjected to radiation which is modified in some physical way depending on the constituents of the liquid. This modification is measured and is an indication of the presence or absence and quantity of a particular analyte.
Of the many optical cells designed for prior analytical systems, many have been designed to measure either the absorbance of a beam of radiation passing through a sample liquid or the turbidity of the sample liquid. Others, using nephelometric techniques measure the amount of radiation that is diffracted 90.degree. to the angle of the original radiation or the fluorescence of the analyte being measured. Unfortunately many of these cells are not designed to measure more than the one physical property efficiently.
One of the cells which is typical of the prior art cells that are capable of measuring the absorbance or turbidity of the sample analyte is described in U.S. Pat. No. 4,330,206 issued May 18, 1982 assigned to Carl Zeiss-Stiftung. This patent describes a optical cuvette which is inherently self-cleaning of air or gas bubbles in liquid samples but again is limited only to the two types of measurements. Still another optical cell of the prior art is described in U.S. Pat. No. 4,440,497 issued Apr. 3, 1984 and assigned to Corning Glass Works. This particular cell is adapted to measure both absorbance and fluorescence of a sample using a single excitation source for the measurement of both. Separate detectors are used for measuring the respective absorbance and fluorescence properties, but the sample is exposed to the atmosphere and its volume is uncontrolled.
In addition to being able to measure using the same cell the four physical properties cited above, it would be highly desirable for a cell to have a maximum absorbance path length in order to achieve maximum sensitivity. To measure fluorescence using the same cell, the entrance slit of the photometer should be fully illuminated by and integrally coupled to the cells' fluorescing liquid. The fluorescent radiation from a sample cell normally is less than that produced by other physical phenomenon; hence, a larger portion of the sample must be excited by the exciting radiation in order to improve the signal to noise ratio of the detector. This creates a problem in that in order to build a cell having sufficient exposure of the sample liquid to the exciting radiation, the cell becomes oversized creating dead spaces so that the ability to measure flowing samples accurately is greatly decreased.
An additional factor in cell design is that the cell should have a small internal volume to satisfy the limited volume of samples that are available from the various analytical chemistries to which the sample is subjected. Finally, the cell should have an internal geometry which allows for relatively efficient wash-out of the samples. And lastly, but not least, the cell should be easy to fabricate and have as low a cost as possible.